Use of cardiac parameters in methods and systems for treating a chronic medical condition

FIELD OF THE INVENTION

This invention relates generally to medical device systems and, more particularly, to medical device systems capable of determining and/or treating a chronic medical condition.

DESCRIPTION OF THE RELATED ART

Many advancements have been made in treating diseases such as depression and epilepsy. Therapies using electrical signals for treating these diseases have been found to be effective. Implantable medical devices have been effectively used to deliver therapeutic stimulation to various portions of the human body (e.g., the vagus nerve) for treating these diseases. As used herein, “stimulation,” “neurostimulation,” “stimulation signal,” or “neurostimulation signal” refers to the application of an electrical, mechanical, magnetic, electromagnetic, photonic, audio, and/or chemical signal to a neural structure in the patient's body. The signal is an exogenous signal that is distinct from the endogenous electrical, mechanical, and chemical activity (e.g., afferent and/or efferent electrical action potentials) generated by the patient's body and environment. In other words, the stimulation signal (whether electrical, mechanical, magnetic, electromagnetic, photonic, audio or chemical in nature) applied to the nerve in the present invention is a signal applied from an artificial source, e.g., a neurostimulator.

A “therapeutic signal” refers to a stimulation signal delivered to a patient's body with the intent of treating a medical condition by providing a modulating effect to neural tissue. The effect of a stimulation signal on neuronal activity is termed “modulation”; however, for simplicity, the terms “stimulating” and “modulating”, and variants thereof, are sometimes used interchangeably herein. In general, however, the delivery of an exogenous signal itself refers to “stimulation” of the neural structure, while the effects of that signal, if any, on the electrical activity of the neural structure are properly referred to as “modulation.” The modulating effect of the stimulation signal upon the neural tissue may be excitatory or inhibitory, and may potentiate acute and/or long-term changes in neuronal activity. For example, the “modulating” effect of the stimulation signal to the neural tissue may comprise one more of the following effects: (a) initiation of an action potential (afferent and/or efferent action potentials); (b) inhibition or blocking of the conduction of action potentials, whether endogenous or exogenously induced, including hyperpolarizing and/or collision blocking, (c) affecting changes in neurotransmitter/neuromodulator release or uptake, and (d) changes in neuro-plasticity or neurogenesis of brain tissue.

In some embodiments, electrical neurostimulation may be provided by implanting an electrical device, i.e., an implantable medical device (IMD), underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. Generally, electrical neurostimulation signals that perform neuromodulation are delivered by the IMD via one or more leads, although leadless neurostimulators have also been developed. The leads generally terminate at their distal ends in one or more electrodes, and the electrodes, in turn, are electrically coupled to tissue in the patient's body. For example, a number of electrodes may be attached to various points of a nerve or other tissue inside a human body for delivery of a neurostimulation signal.

While feedback stimulation (i.e., an electrical signal applied in response to a sensed body parameter such as heart rate) schemes have been proposed, conventional vagus nerve stimulation (VNS) usually involves non-feedback stimulation characterized by a number of parameters. Specifically, conventional vagus nerve stimulation usually involves a series of grouped electrical pulses defined by an “on-time” and an “off-time.” Each sequence of pulses during an on-time may be referred to as a “pulse burst.” The burst is followed by the off-time period in which no signals are applied to the nerve. During the on-time, electrical pulses of a defined electrical current (e.g., 0.5-2.0 milliamps) and pulse width (e.g., 0.25-1.0 milliseconds) are delivered at a defined frequency (e.g., 20-30 Hz) for the on-time duration, usually a specific number of seconds, e.g., 10-60 seconds. The pulse bursts are separated from one another by the off-time, (e.g., 30 seconds-5 minutes) in which no electrical signal is applied to the nerve. The on-time and off-time parameters together define a duty cycle, which is the ratio of the on-time to the combination of the on-time and off-time, and which describes the percentage of time that the electrical signal is applied to the nerve.

In conventional VNS, the on-time and off-time may be programmed to define an intermittent pattern in which a repeating series of electrical pulse bursts are generated and applied to a cranial nerve such as the vagus nerve. The off-time is provided to allow the nerve to recover from the stimulation of the pulse burst, and to conserve power. If the off-time is set at zero, the electrical signal in conventional VNS may provide continuous stimulation to the vagus nerve. Alternatively, the off time may be as long as one day or more, in which case the pulse bursts are provided only once per day or at even longer intervals. Typically, however, the ratio of “off-time” to “on-time” may range from about 0.5 to about 10.

In addition to the on-time and off-time, the other parameters defining the electrical signal in conventional VNS may be programmed over a range of values. The pulse width for the pulses in a pulse burst of conventional VNS may be set to a value not greater than about 1 msec, such as about 250-500 μsec, and the number of pulses in a pulse burst is typically set by programming a frequency in a range of about 20-150 Hz (i.e., 20 pulses per second to 150 pulses per second). A non-uniform frequency may also be used. Frequency may be altered during a pulse burst by either a frequency sweep from a low frequency to a high frequency, or vice versa. Alternatively, the timing between adjacent individual signals within a burst may be randomly changed such that two adjacent signals may be generated at any frequency within a range of frequencies.

Although neurostimulation has proven effective in the treatment of a number of medical conditions, it would be desirable to further enhance and optimize neurostimulation for this purpose. For example, it may be desirable to further enhance treatment of chronic medical conditions. Generally, the state of the art implantable medical devices lack efficient treatment based on certain feedback from the patient's body for treating chronic conditions. This may be due to either a belief that no reliable body parameter is a reliable indicator or predictor of the disease state of the chronic medical condition; the difficulty of isolating a particular indicator from many factors that contribute to the disease state; a belief that feedback systems will provide no benefit over non-feedback treatment for chronic medical conditions; or to combinations of the foregoing. In sharp contrast to this general neglect of feedback-based treatments for chronic illnesses, designers have proposed numerous systems to provide treatment of certain episodic or acute medical conditions based upon feedback of certain body parameters. For example, many different feedback schemes for detection and treatment of epileptic seizures have been proposed, including feedback based upon EEG signals, heart rate, and sensing of neural activity. Further, the relative shortage of reliable treatments for many chronic medical conditions may also contribute to the absence of feedback-based treatment options for chronic medical conditions. This lack of sufficient knowledge as to the treatment of chronic or non-episodic type ailments presents difficult challenges in analyzing body parameters to provide feedback-based treatment for such conditions. There is a need for improved treatment options, including feedback-based treatment options, for many chronic medical conditions.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method for treating a chronic medical condition in a patient is provided. A time of beat sequence of the patient's heart is determined. A regulatory system parameter is determined based on the time of beat sequence. The parameter is indicative of a stress level of the patient's regulatory adaptation systems. The determined regulatory system parameter is compared with a threshold regulatory system parameter value. An electrical signal is applied to a neural structure of the patient to treat the chronic medical condition if the determined regulatory system parameter exceeds the threshold regulatory system parameter value.

In another aspect of the present invention, another method for treating a chronic medical condition in a patient is provided. A time of beat sequence of the patient's heart is determined. A heart rate variability (HRV) parameter is determined based on the time of beat sequence. The HRV parameter is associated with a parasympathetic function of the patient. The determined HRV parameter is compared with a threshold HRV parameter value. An electrical signal is applied to a neural structure to treat the chronic medical condition if the determined HRV parameter exceeds the threshold HRV value.

In another aspect of the present invention, an implantable medical device (IMD) for treating a chronic medical condition in a patient is provided. The IMD comprises a sensing module for sensing data relating to a time of beat sequence of the patient's heart and a heart beat data processing module to process the data relating to the time of beat sequence. The IMD also includes a heart parameter module adapted to determine at least one heart parameter selected from a heart rate variability (HRV) parameter associated with a parasympathetic function of the patient and a regulatory system parameter indicative of a stress level of the patient's regulatory adaptation systems. The heart parameter module is also adapted to determine the at least one heart parameter based upon the processed data relating to the time of beat sequence of the patient's heart. The IMD also includes a comparator adapted to compare the heart parameter determined by the heart parameter module with a threshold heart parameter value. The IMD also includes an electrical signal module adapted to generate and apply an electrical signal to a neural structure to treat the chronic medical condition in the patient based upon the comparison of the determined heart parameter value and the threshold heart parameter value.

In yet another aspect of the present invention, a computer readable program storage device is provided that is encoded with instructions that, when executed by a computer, perform a method for treating a chronic medical condition in a patient is provided. The method includes determining a time of beat sequence of the patient's heart and determining a first index of regulatory activity systems (IARS) value associated with the patient based upon the time of beat sequence. The method also includes comparing the first IARS value to a first threshold IARS value and providing a therapy to the patient in response to a determination that the first IARS value exceeds the first threshold IARS value.

In yet another aspect of the present invention, a computer readable program storage device is provided that is encoded with instructions that, when executed by a computer, perform a method for treating a chronic medical condition in a patient is provided. The method includes determining a time of beat sequence of the patient's heart and determining an HRV parameter indicative of parasympathetic activity from the time of beat sequence. The method also includes comparing the determined HRV value to a threshold HRV value; and providing a therapy to the patient in response to a determination that the HRV value is below the threshold HRV value.

In yet another aspect of the present invention, an implantable medical device system is provided for providing an indication of a level of stress upon a regulatory system of a patient having a chronic medical condition. The system includes an implantable medical device (IMD) that comprises a sensing module for sensing data relating to a time of beat sequence of the patient's heart and a heart beat data processing module adapted to process the data relating to the time of beat sequence. The IMD also includes a regulatory activity module adapted to determine an index of regulatory activity systems (IARS) value associated with the patient based upon the data relating to the time of beat sequence of the patient's heart. The IARS value is indicative of level of stress upon the regulatory system of the patient. The IMD also includes a communication module adapted to provide the IARS value to at least one of a patient or a healthcare provider. The medical device is adapted to provide a treatment to the patient in response to a determination that the IRS value indicates a chronic condition in the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:

<figref idrefs="DRAWINGS">FIG. 1A</figref> provides a stylized diagram of an implantable medical device implanted into a patient's body for providing a therapeutic electrical signal to a neural structure of the patient's body, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 1B</figref> provides a stylized diagram of an implantable medical device implanted into a patient's body for providing a therapeutic electrical signal to a neural structure of the patient's body, in accordance with another illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 2</figref> is a block diagram of a medical device system that includes a medical device and a monitoring device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 3</figref> provides a stylized diagram of an implantable medical device implanted into a patient's body for providing a therapeutic electrical signal to a neural structure of the patient's body, in accordance with yet another illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 4A</figref> illustrates a stylized block diagram of a heart beat sensor interface of a medical device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 4B</figref> illustrates a stylized block diagram of a heart beat data processing module of a medical device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 5A</figref> illustrates a block diagram of a heart rate variability (HRV) module of the implantable medical device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 5B</figref> illustrates a block diagram of a regulatory state module of the implantable medical device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 6</figref> illustrates a flowchart relating to a function performed by a heart beat data processing module of <figref idrefs="DRAWINGS">FIG. 2</figref>, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 7</figref> illustrates a block diagram depiction of a heart rhythm regulation module, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 8</figref> illustrates a flowchart depiction of steps for treating a chronic medical condition using a medical device, in accordance with one illustrative embodiment of the present invention;

<figref idrefs="DRAWINGS">FIG. 9</figref> illustrates a flowchart depiction of treating a chronic condition using an IARS parameter, in accordance with one illustrative embodiment of the present invention; and

<figref idrefs="DRAWINGS">FIG. 10</figref> illustrates a flowchart depiction of treating a chronic condition using an HRV parameter in accordance with one illustrative embodiment of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described herein. In the interest of clarity, not all features of an actual implementation are described in this specification. In the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the design-specific goals, which will vary from one implementation to another. It will be appreciated that such a development effort, while possibly complex and time-consuming, would nevertheless be a routine undertaking for persons of ordinary skill in the art having the benefit of this disclosure.

This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “includes” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” Also, the term “couple” or “couples” is intended to mean either a direct or an indirect electrical connection. “Direct contact,” “direct attachment,” or providing a “direct coupling” indicates that a surface of a first element contacts the surface of a second element with no substantial attenuating medium there between. The presence of small quantities of substances, such as bodily fluids, that do not substantially attenuate electrical connections does not vitiate direct contact. The word “or” is used in the inclusive sense (i.e., “and/or”) unless a specific use to the contrary is explicitly stated.

The term “electrode” or “electrodes” described herein may refer to one or more stimulation electrodes (i.e., electrodes for delivering an electrical signal generated by an IMD to a tissue), sensing electrodes (i.e., electrodes for sensing a physiological indication of a patient's body), and/or electrodes that are capable of delivering a stimulation signal, as well as performing a sensing function.

Cranial nerve stimulation has been proposed to treat a number of medical conditions pertaining to or mediated by one or more structures of the nervous system of the body, including epilepsy and other movement disorders, depression, anxiety disorders and other neuropsychiatric disorders, dementia, traumatic brain injury, coma, migraine headache, obesity, eating disorders, sleep disorders, cardiac disorders (such as congestive heart failure and atrial fibrillation), hypertension, endocrine disorders (such as diabetes and hypoglycemia), and pain (including neuropathic pain and fibromyalgia), among others. See, e.g., U.S. Pat. Nos. 4,867,164; 5,299,569; 5,269,303; 5,571,150; 5,215,086; 5,188,104; 5,263,480; 6,587,719; 6,609,025; 5,335,657; 6,622,041; 5,916,239; 5,707,400; 5,231,988; and 5,330,515. Despite the numerous disorders for which cranial nerve stimulation has been proposed or suggested as a treatment option, the fact that detailed neural pathways for many (if not all) cranial nerves remain relatively unknown, makes predictions of efficacy for any given disorder difficult or impossible. Moreover, even if such pathways were known, the precise stimulation parameters that would modulate particular pathways relevant to a particular disorder generally cannot be predicted.

In some embodiments, electrical neurostimulation may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. In another alternative embodiment, the signal may be generated by an external pulse generator outside the patient's body, coupled by an RF or wireless link to an implanted electrode.

Embodiments of the present invention provide for treating a chronic or a non-episodic condition using heart beat signals. In one embodiment, heart beat signals indicative of heart rate variability (HRV) (“HRV signals”) are used to treat chronic or non-episodic medical condition(s). In one embodiment, HRV signals may be used to treat chronic or non-episodic conditions that adversely affect the parasympathetic system of a patient's body.

HRV may be related to the degree of tension of the regulatory systems (or regulatory adaptation system) affected by the activation of the pituitary-adrenal system. This activation may occur in response to a stress and/or in reaction to a response of the sympathoadrenal system. Under stress, a person having a sufficient functional reserve would respond with a normal working tension or stress of the regulatory system. For this person, in rest, the tension or stress of the regulatory system would be relatively low. However, a person with a medical condition, and particularly a chronic medical condition, may display a relatively high tension of the regulatory system, even under situations that, absent the patient's medical condition, would be non-stressful, e.g., rest or mild activity. HRV may also refer to a functional system (described in further detail below) that relates to numerous regulatory mechanisms affecting the circulatory system (e.g., regulation of blood circulation). One HRV variable or regulatory system parameter useful in embodiments of the present invention is the index of activity of regulatory system (IARS), which relates to values reflecting different degrees of tension of the regulatory system and the adaptation ability of a patient's body.

As an illustrative example of an embodiment of the present invention, HRV signals may be analyzed using various methods to decipher a state of the regulatory system of a patient's body. The “state” may refer to the stability or instability of a patient's regulatory system, which may relate to the stress upon the regulatory system. Based upon the state of the regulatory system of a patient's body, a determination may be made that a particular treatment may be provided to treat a chronic condition. Chronic conditions that are treatable by embodiments of the present invention include, but are not limited to, congestive heart failure, traumatic brain injury (TBI), depression, stroke, fibromyalgia, obesity, etc. These diseases, although they occasionally may manifest certain episodic instances, are generally chronic diseases that are related to the regulatory system. Particularly, these diseases may be associated with compromised functioning of at least a portion of the parasympathetic nervous system of a patient.

The HRV signals can be gathered by any of a number of techniques. For example, data relating to a beat sequence (or HRV signals) may be gathered by an electrocardiogram (ECG) device. In one embodiment, the data relating to the beat sequence (or HRV signals) may be related to the R-waves of the beat sequence, such as a time series of R-waves or a series of R-R intervals. Those skilled in the art having the benefit of the present disclosure would appreciate that other time series of waves may be used and still remain within the spirit and scope of the present invention.

Data relating to R-waves may be gathered by an ECG device or, in one embodiment, by a vagus nerve stimulator, such as described in U.S. patent application Ser. No. 12/258,019, filed Oct. 24, 2008, which is hereby incorporated by reference herein.

The R-R interval variations of a patient's heart beat are influenced by vagal activity. Moreover, the vagal activity may prompt inhibitions of sympathetic nervous system activity. In other words, utilizing information present in HRV data, influences upon the sympathetic and the parasympathetic activity of the human body may be determined. In treatment of chronic conditions, such as diabetes, a reduction in value of the time domain parameters of the HRV may carry a negative prognostic value. Thus, the value of the time domain parameters of the HRV, as well as other HRV parameters described herein, may be used as indicator(s) to characterize various chronic conditions, such as diabetes. Further, variance in the absolute power of the low frequency (LF) and the high frequency (HF) values in diabetic patients may also exist, and the LF:HF ratio may be indicative of a negative prognostic value.

Further, value of parameter resulting from statistical analysis of HRV data may also be indicative of one or more chronic conditions. Using a series of instantaneous heart rates or intervals of heart beat cycle, statistical measures relating to HRV may be determined. For example, statistical time domain HRV parameters may be calculated using direct measurements of normal-to-normal (NN) intervals or from the differences between NN intervals. One example of a statistical time domain parameter is the standard deviation of the NN intervals, i.e., the square root of the variance between NN intervals. These types of HRV parameters are described in further details below.

Moreover, geometric methods of analyzing HRV data may also be performed to determine if the HRV data indicates a chronic condition. For example, NN intervals may be converted into a geometric pattern. These patterns may include sample density of distribution of NN intervals durations, sample density distributions of differences between adjacent NN intervals, Lorenz plot of NN or RR intervals, etc. Geometric analysis of HRV data may include converting a measurement of the geometric pattern (e.g., width of the distribution histogram) into an HRV parameter, interpolating a geometric pattern using mathematic shape defining techniques (e.g., approximation of the distribution histogram using a triangle method or an exponential curve) to define an HRV parameter, and/or classifying the geometric shape into pattern based categories (e.g., elliptic, linear, triangular shapes of Lorentz plots) that represent different types of heart rate variations. The results from applying geometric methods for analyzing HRV data may provide indications of one or more chronic conditions in a patient.

Spectral components of the HRV may also be relatively low for cardiac patients and, therefore, spectral analysis of the HRV resulting in low volumes of parameters may be indicative of heart conditions. In other words, a reduced HRV parameter value may be indicative of compromised cardiac functioning, or of a rising level of stress upon the cardiac regulatory systems. The LF component in the spectral analysis, for example, may be relatively low in cardiac patients. Oscillations in the spectral LF components of the HRV may be indicative of chronic cervical spinal cord disorders, e.g., lesions. These are examples of variations in the HRV components that may be detected, and this information may be used to treat various chronic conditions, such as depression, anxiety disorders and other neuropsychiatric disorders, dementia, traumatic brain injury, coma, obesity, chronic cardiac disorders (such as congestive heart failure), hypertension, chronic endocrine disorders (such as diabetes), and chronic pain (including neuropathic pain and fibromyalgia), among others. Utilizing embodiments of the present invention, treatment of chronic conditions, particularly chronic conditions that adversely affect the parasympathetic system of the patient, may be performed. Influences upon the regulatory system of the patient may be detected based upon HRV data, and appropriate therapy may be applied to reduce and treat one or more chronic conditions.

Receiving the data relating to the beat sequence of the patient's heart (or “HRV signals” per previous discussion) may comprise sensing a time of beat sequence of a patient's heart and generating a time series data stream from said time of beat sequence. In a further embodiment, receiving the data relating to the beat sequence of the patient's heart may comprise receiving a series of R-R intervals, and generating the time series data stream may comprise sensing a plurality of R peaks from the R-R intervals and using the R peaks for providing time stamps to generate the time series data stream based upon the time stamps.

From the R-R interval data, various classes of HRV parameters may be determined. Known classes include time domain HRV parameters, which may include statistical parameters (e.g., PNN50, SDNN, SDANN, RMSSD and logarithmic index), geometric HRV parameters (such as those derived from Lorenz plots); frequency domain HRV parameters, which primarily involve spectral parameters derived from power spectral density analysis (such as the high frequency (HF), low frequency (LF, very low frequency (VLF), and ultra low frequency (ULF) components, and various ratios of the foregoing); rhythm pattern HRV parameters, such is the interval spectrum and spectrum of counts; and nonlinear HRV parameters, which include parameters derived from Fourier spectra analysis, H scaling exponents, Coarse Graining Spectral Analysis, Poincare sections, low-dimension attractor plots, singular value decomposition, attractor trajectories, D2 correlation dimension, Lyapunov exponents, and Kolmogorov entropy. Summaries of different HRV parameters are widely available in the cardiology literature. See, e.g., the 1996 publication by the Task Force of the European Society of Cardiology and the North American Society of Pacing Electrophysiology entitled “Heart Rate Variability, Standards of Measurement, Physiological Interpretation and Clinical Use.”

Without being bound by theory, it is believed that vagal activity influences a variety of aspects of chronic medical conditions through a variety of mechanisms, such as modulation of neurotransmitter production and effects upon inflammatory cytokine production. In the present invention, HRV parameters conveying information about vagal activity and function, including its relative activity as compared to sympathetic neural function, are those primarily intended for incorporation into embodiments of the present invention. Many other HRV parameters from the foregoing broad classes of HRV parameters are indicative of one or more aspects of sympathetic and/or parasympathetic functioning, and it should be appreciated that those specifically listed here are non-limiting examples.

One useful sub-class of time domain HRV parameters includes statistical parameters, which may be calculated easily from heart beat data and which provide many parameters useful in embodiments of the present invention. Based upon the heartbeat data, the QRS complex may be evaluated and normal-to-normal (NN) intervals may be determined. Normal-to-normal intervals may refer to intervals between adjacent corresponding QRS complexes (respectively). Using the NN intervals, a statistical measure that results in a percentage parameter relating to an NN 50 count divided by the total number of all NN intervals, may be determined, i.e., PNN50. In an alternative embodiment, PNN50 may refer to the number of NN interval pairs with a difference exceeding 50 ms, divided by the total number of NN intervals. The PNN50 value may be indicative of the degree of the predominance of the parasympathetic (i.e., vagal) portion of the patient's autonomic regulation over the sympathetic portion of the autonomic regulation.

Further, using the NN intervals, another statistical measure that results in a time parameter (ms) using a standard deviation of all NN intervals, may be determined, i.e., SDNN. The SDNN value may relate to the total effect of the autonomic regulation of blood circulation of the patient's body.

Moreover, the square root of the sum of differences of a sequential series of the NN intervals may also be calculated to provide RMSSD. The RMSSD may relate to the activity of the parasympathetic portion of the autonomic regulation of a patient's body.

Spectral HRV parameters comprise another class of HRV parameters yielding useful information for embodiments of the present invention. One example of the spectral parameter may include a low frequency (LF) power variability component. This component may be expressed as percentage of the low-frequency power variability in relation to the total power fluctuations. Another example of the spectral parameter may include a high frequency (HF) power variability component. The HF power component is generally regarded as an indication of vagal activity. This component may be expressed as percentage of the high-frequency power variability in relation to the total power fluctuations. Further a frequency ratio may be determined by calculating a ratio of the low frequency and high frequency power components with the magnification of a central frequency of each component.

Nonlinear HRV parameters may also provide information useful in embodiments of the present invention. In addition to the parameters described above, other non-linear parameters exist and may be strongly correlated, either positively or negatively, with one or more of the above-referenced time domain and/or spectral domain HRV parameters. However, it must be borne in mind that many of the other parameters that exist have relatively weak or essentially no correlation with any of the above parameters. In general, HRV parameters, regardless of which class or sub-class they are part, are useful in the present invention when they possess a relatively high correlation with other parameters known to be indicative of one or more aspects of parasympathetic function or its relationship to sympathetic function. It should be noted that whether any two HRV parameters have a strong correlation, a weak correlation, or essentially no correlation cannot be determined a priori for any particular medical condition, and instead must be derived from empirical observation and application of concepts provided by embodiments of the present invention. It must further be borne in mind that variability in the value of a parameter over a period of time has no a priori correlation with the patient's heart beat over the same period of time.

In order to treat a chronic or non-episodic condition, an index of activity of regulatory systems (IARS) value for a particular patient may be determined. IARS is a parameter that was developed in the context of space medicine to study the effects of space on the human body. More particularly, there was a desire to study the stress on a person's regulatory system due to travel in space. The prior analysis of IARS was directed to the study of stress of regulatory system in relation to pre-nosologic states. That is, prior application and studies of IARS was directed to non-disease contexts to analyze the effects of space travel on the human body. Utilizing embodiments of the present invention, IARS values may be used to provide feedback and/or treatment of chronic or non-episodic conditions. Specific IARS value(s) of a patient may be determined to determine the presence and/or severity of a chronic or non-episodic condition of a patient. Further, a predetermined threshold IARS value for that patient may be determined. In other embodiments, the threshold IARS value may be a dynamic value that may be changed over time. The threshold IARS can be established by a physician based on one or more of the particular parameter(s) under consideration, the patient's age, the patient's sex, the condition of the patient's heart, the severity of the patient's medical disorder, a blood parameter, health status of the patient, IARS history of the patient, and/or other parameters.

Based on a comparison of the determined IARS valued to a predetermined threshold value, a treatment action for the chronic medial condition may be performed. Embodiments of the present invention provide for one or more types of treatment therapies that may be provided to treat the chronic condition. These treatments may include applying an electrical signal to at least a brain structure, a cranial nerve, a spinal cord of a patient, a sympathetic nerve, and/or a peripheral nerve of a patient. The application of the treatment may also include an analysis wherein an HRV parameter (e.g., PNN50) is below a predetermined threshold. The processing of the HRV signal includes analysis of the IARS or the value relating to the HRV signal. These analyses provide for an indication that treatment of a chronic condition may be desirable. Based upon this indication, various treatment regimens may be implemented by the IMD to treat the chronic or the non-episodic condition.

Treatment for a chronic condition may include one of a plurality of types of electrical signal, wherein various characteristics of the electrical signal (e.g., frequency, pulse-width, on-time, off-time, amplitude, etc.) may be controlled based upon the type and severity of the chronic condition. Further, other types of treatment, e.g., drugs and/or cooling of a neural structure of the patient, may be implemented in addition or alternative to the electrical signal therapy described herein. The particular type of delivery of drug treatments, the type of drugs and/or cooling of a neural structure may be provided by those skilled in the art having benefit of the present disclosure.

Although not so limited, a system capable of implementing embodiments of the present invention is described below. <figref idrefs="DRAWINGS">FIG. 1A</figref> depicts a stylized implantable medical system (IMD) <b>100</b> for implementing one or more embodiments of the present invention. An electrical signal generator <b>110</b> is provided, having a main body <b>112</b> comprising a case or shell with a header <b>116</b> for connecting to an insulated, electrically conductive lead assembly <b>122</b>. The generator <b>110</b> is implanted in the patient's chest in a pocket or cavity formed by the implanting surgeon just below the skin (indicated by a dotted line <b>145</b>), similar to the implantation procedure for a pacemaker pulse generator.

A nerve electrode assembly <b>125</b>, preferably comprising a plurality of electrodes having at least an electrode pair, is conductively connected to the distal end of the lead assembly <b>122</b>, which preferably comprises a plurality of lead wires (one wire for each electrode). Each electrode in the electrode assembly <b>125</b> may operate independently or alternatively, may operate in conjunction with the other electrodes. In one embodiment, the electrode assembly <b>125</b> comprises at least a cathode and an anode. In another embodiment, the electrode assembly comprises one or more unipolar electrodes.

Lead assembly <b>122</b> is attached at its proximal end to connectors on the header <b>116</b> of generator <b>110</b>. The electrode assembly <b>125</b> may be surgically coupled to the vagus nerve <b>127</b> in the patient's neck or at another location, e.g., near the patient's diaphragm or at the esophagus/stomach junction. Other (or additional) cranial nerves such as the trigeminal and/or glossopharyngeal nerves may also be used to deliver the electrical signal in particular alternative embodiments. In one embodiment, the electrode assembly <b>125</b> comprises a bipolar stimulating electrode pair <b>126</b>, <b>128</b> (i.e., a cathode and an anode). Suitable electrode assemblies are available from Cyberonics, Inc., Houston, Tex., USA as the Model 302 electrode assembly. However, persons of skill in the art will appreciate that many electrode designs could be used in the present invention. In one embodiment, the two electrodes are wrapped about the vagus nerve, and the electrode assembly <b>125</b> may be secured to the vagus nerve <b>127</b> by a spiral anchoring tether <b>130</b> such as that disclosed in U.S. Pat. No. 4,979,511 issued Dec. 25, 1990 to Reese S. Terry, Jr. and assigned to the same assignee as the instant application. Lead assembly <b>122</b> may be secured, while retaining the ability to flex with movement of the chest and neck, by a suture connection to nearby tissue (not shown).

In some embodiments, the electrode assembly <b>125</b> may comprise temperature sensing elements and/or heart beat sensor elements. Other sensors for other body parameters may also be employed. Both passive and active stimulation may be combined or delivered by a single IMD according to the present invention. Either or both modes may be appropriate to treat a specific patient under observation.

In alternative embodiments, the implantable medical device system further comprises an electrical stimulator comprising an electrode <b>160</b> (not to scale) adapted to be coupled to the spinal cord <b>180</b> (<figref idrefs="DRAWINGS">FIG. 1B</figref>) or to a region of the brain <b>190</b> (<figref idrefs="DRAWINGS">FIG. 3</figref>). The physician can select precise locations for coupling to the spinal cord <b>180</b> or brain <b>190</b> based on his or her observations of the patient's medical condition, among other values. In various embodiments, the implantable medical device system may comprise one, two, or three of the IMD <b>100</b>, the spinal cord stimulator, and the brain stimulator. The system <b>100</b> may also comprise a cooling apparatus <b>148</b> (<figref idrefs="DRAWINGS">FIG. 3</figref>) that is capable of cooling a neural structure of the patient.

The electrical pulse generator <b>110</b> may be programmed with an external device (ED) such as computer <b>150</b> using programming software known in the art. A programming wand <b>155</b> may be coupled to the computer <b>150</b> as part of the ED to facilitate wireless radio frequency (RF) communication between the computer <b>150</b> and the implanted pulse generator <b>110</b>. The programming wand <b>155</b> and computer <b>150</b> permit non-invasive communication with the generator <b>110</b> after the latter is implanted. In systems where the computer <b>150</b> uses one or more channels in the Medical Implant Communications Service (MICS) bandwidths, the programming wand <b>155</b> may be omitted to permit more convenient communication directly between the computer <b>150</b> and the pulse generator <b>110</b>.

Turning now to <figref idrefs="DRAWINGS">FIG. 2</figref>, a block diagram depiction of the medical device (MD) <b>200</b> is provided, in accordance with one illustrative embodiment of the present invention. The MD <b>200</b> (such as implantable generator <b>110</b> from <figref idrefs="DRAWINGS">FIG. 1</figref>) may comprise a controller <b>210</b> capable of controlling various aspects of the operation of the MD <b>200</b>. The controller <b>210</b> is capable of receiving data and causing a stimulation unit <b>220</b> to generate and deliver an electrical signal to target tissues of the patient's body for treating a medical condition. For example, the controller <b>210</b> may receive instructions from another device such as Monitoring Unit <b>270</b>, or may cause the electrical signal to be generated and delivered based on calculations and programming internal to MD <b>200</b>. The controller <b>210</b> is capable of affecting substantially all functions of the MD <b>200</b>. The MD <b>200</b> may be an external device, or in an alternative embodiment, an implantable medical device.

The controller <b>210</b> may comprise various components, such as a processor <b>215</b>, a memory <b>217</b>, etc. The processor <b>215</b> may comprise one or more microcontrollers, microprocessors, etc., capable of performing various executions of software components. The memory <b>217</b> may comprise various memory portions where a number of types of data (e.g., internal data, external data instructions, software codes, status data, diagnostic data, etc.) may be stored. The memory <b>217</b> may comprise one or more of random access memory (RAM), dynamic random access memory (DRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, etc.

The MD <b>200</b> may also comprise a stimulation unit <b>220</b> capable of generating and delivering electrical signals to one or more electrodes <b>126</b>, <b>128</b> via a coupling <b>201</b>. In one embodiment, MD <b>200</b> may be an implantable medical device, and coupling <b>201</b> may comprise a lead assembly such as lead assembly <b>122</b> (<figref idrefs="DRAWINGS">FIG. 1</figref>). In another embodiment, MD <b>200</b> may be external to the patient's body, and may be coupled to an implanted lead via an inductive coupling such as an RF inductive coupling. Whether MD <b>200</b> is an implantable or external unit, a therapeutic electrical signal may be delivered to the electrodes <b>226</b>, <b>228</b> by the stimulation unit <b>220</b> based upon instructions from the controller <b>210</b>. The stimulation unit <b>220</b> may comprise various circuitry, such as electrical signal generators, impedance control circuitry to control the impedance “seen” by the leads, and other circuitry that receives instructions relating to the delivery of the electrical signal to tissue.

In other embodiments, coupling <b>201</b> is operatively coupled to an electrode, wherein the electrode is adapted to couple to at least one of a portion of a brain structure of the patient, a cranial nerve of a patient, a spinal cord of a patient, a sympathetic nerve structure of the patient, or a peripheral nerve of the patient.

The MD <b>200</b> may also comprise a power supply <b>230</b>. The power supply <b>230</b> may comprise a battery, voltage regulators, capacitors, etc., to provide power for the operation of the MD <b>200</b>, including delivering the therapeutic electrical signal. The power supply <b>230</b> comprises a power source that in some embodiments may be rechargeable. In other embodiments, a non-rechargeable power source may be used. The power supply <b>230</b> provides power for the operation of the MD <b>200</b>, including electronic operations and the electrical signal generation and delivery functions. The power supply <b>230</b> may comprise a lithium/thionyl chloride cell or a lithium/carbon monofluoride (LiCFx) cell for implantable embodiments, and more common watch batteries or 9 volt batteries for non-implantable embodiments. Other battery types known in the art of implantable medical devices may also be used.

The MD <b>200</b> may also comprise a communication unit <b>260</b> capable of facilitating communications between the MD <b>200</b> and various devices. In particular, the communication unit <b>260</b> is capable of providing transmission and reception of electronic signals to and from a monitoring unit <b>270</b>, such as computer <b>150</b> and a wand <b>155</b> that can communicate with the MD <b>200</b> remotely (<figref idrefs="DRAWINGS">FIG. 1</figref>). The communication unit <b>260</b> may include hardware, software, firmware, or any combination thereof.

The MD <b>200</b> may also comprise one or more sensor(s) <b>212</b> coupled via sensor coupling <b>211</b> (which may comprise a lead or an inductive coupling) to the MD <b>200</b>. The sensor(s) <b>212</b> are capable of receiving signals related to a physiological parameter, such as the patient's heart beat, and delivering the signals to the MD <b>200</b>. In one embodiment, the sensor(s) <b>212</b> may be the same as electrode(s) <b>226</b>, <b>228</b>. In other embodiments, the sensor(s) <b>212</b> are separate structures that may be placed in, on, or near the patient's heart, or outside the patient's skin, such as over the patient's heart or elsewhere on the patient's torso.

In one embodiment, the MD <b>200</b> may comprise a heart beat sensor interface <b>265</b> that is capable of receiving signals related to the patient's heart beat from the sensor(s) <b>212</b>. The heart beat sensor interface <b>265</b> may be capable of performing any necessary or suitable amplifying, filtering, and performing analog-to-digital (A/D) conversions to prepare the signals for downstream processing. The heart beat sensor interface, in one embodiment, may comprise software module(s) that are capable of performing various interface functions, filtering functions, etc., to process heart beat signals. In another embodiment the heart beat sensor interface <b>265</b> may comprise hardware circuitry that is capable of performing these functions. In yet another embodiment, the heart beat sensor interface <b>265</b> may comprise hardware, firmware, software and/or any combination thereof. A more detailed illustration of the heart beat sensor interface <b>265</b> is provided in <figref idrefs="DRAWINGS">FIG. 4</figref> and accompanying description below.

The heartbeat sensor interface <b>265</b> is capable of receiving heartbeat signals and providing the signal to a heart beat data processing module <b>275</b> that the MD <b>200</b> may comprise. Based upon the signals processed by the heart beat sensor interface <b>265</b>, a heart beat data processing module <b>275</b> may determine various properties of the patient's heart beat time series and store such properties or forward them on for further processing/analysis. In one embodiment, the heart beat data processing module <b>275</b> is capable of processing the heart beat into various components such as those necessary to determine HRV parameters and/or parameters indicative of stress upon a regulatory system. For example, the heart beat data processing module <b>275</b> is capable of processing the heart beat signals such that IARS values and or HRV parameters may be determined. For example, parameters such as PNN50, SDNN, SDANN, RMSSD, HF spectral power, interval spectrum, and Lyapunov exponents that may be indicative of a chronic or non-episodic physiological event involving vagal function may be determined by the heart beat data processing module <b>275</b>.

Moreover, the MD <b>200</b> may also comprise a heart parameter module <b>284</b>. Two submodules, HRV module <b>285</b> and regulatory state module <b>287</b>, may be provided as part of heart parameter module <b>284</b>. It may be appreciated that heart parameter module <b>284</b> may also comprise other modules for determination of heart parameters in addition to those determined by HRV module <b>285</b> and regulatory state module <b>287</b>. The HRV module <b>285</b> is capable of determining one or more HRV parameters from the data derived from the heart beat signal. Further description of the HRV module <b>285</b> is provided in <figref idrefs="DRAWINGS">FIG. 5A</figref> and accompanying description below.

The MD <b>200</b> also comprises a regulatory state module <b>287</b>. The regulatory state module <b>287</b> is capable of using the data processed by the heart beat data processing module <b>275</b> to determine whether the patient's regulatory adaptation system is under stress or has become unstable. This determination may be made using a variety of algorithms. The regulatory state module <b>287</b> may perform a comparison of a calculated regulatory parameter value such an IARS value to a predetermined or adaptive threshold regulatory parameter value.

The regulatory state module <b>287</b> may perform an adaptive adjustment of the threshold regulatory parameter value based upon various factors, such as various physiological data relating to the patient and environmental data such as time of day and time of year. The physiological data may include the age of the patient, the height, weight, blood pressure, normal heart rate, general health, stress levels, etc. In this manner, a dynamic threshold may be adapted in order to determine whether the regulatory state of the patient is under stress based upon the threshold value. When the regulatory state module <b>287</b> determines that the patient's regulatory system is under excessive stress or has become unstable, the MD <b>200</b> may provide a predetermined therapeutic electrical signal to stimulate at least a portion of the patient's vagus nerve. The electrical signal may be adjusted in such a manner to affect the parasympathetic system of the patient to provide a reduction in the stress level of the regulatory system of the patient, or to reduce the instability of the patient's regulatory system.

In various embodiments, one or more of the units or modules described above may be located in an monitoring unit <b>270</b> or a remote device <b>292</b>, with communications between that unit or module and communication unit <b>260</b> in the MD <b>200</b> taking place via a link <b>277</b>, which may comprise a lead or, more preferably an inductive RF or similar wireless coupling. For example, in one embodiment, the heart parameter module <b>284</b> may be external to the medical device <b>200</b> and to the patient's body, e.g., in a monitoring unit <b>270</b>. Locating the heart parameter module <b>284</b> in a monitoring unit <b>270</b> may be advantageous if the heart parameter calculation is computationally intensive, in order to reduce energy expenditure and heat generation in the MD <b>200</b> or to expedite calculation of the at least one heart parameter.

Monitoring unit <b>270</b> may be capable of displaying an indication to a patient that a heart parameter, such as an HRV parameter or a regulatory state parameter, has exceeded a threshold for that parameter. Monitoring unit <b>270</b> may also be capable of logging the time of the event, as well as initiate a severity determination to indicate the duration and extent of the heart parameter exceeding a threshold, and to notify a caregiver or store the information in one or more databases.

The monitoring unit <b>270</b> may be a device that is capable of programming electrical signal parameters of the MD <b>200</b>. In one embodiment, the monitoring unit <b>270</b> is a computer system capable of executing a data-acquisition program. The monitoring unit <b>270</b> may be controlled by a healthcare provider, such as a physician, at a base station in, for example, a doctor's office. In alternative embodiments, the monitoring unit <b>270</b> may be controlled by a patient in a system providing less control over the operation of the MD <b>200</b> than another monitoring unit <b>270</b> controlled by a healthcare provider. Whether controlled by the patient or by a healthcare provider, the monitoring unit <b>270</b> may be a computer, preferably a handheld computer or PDA, but may alternatively comprise any other device that is capable of electronic communications and programming, e.g., hand-held computer system, a PC computer system, a laptop computer system, a server, a personal digital assistant (PDA), an Apple-based computer system, etc. The monitoring unit <b>270</b> may download various parameters and program software into the MD <b>200</b> for programming the operation of the IMD, and may also receive and upload various status conditions and other data from the MD <b>200</b>. Communications between the monitoring unit <b>270</b> and the communication unit <b>260</b> in the MD <b>200</b> may occur via a wireless or other type of communication, represented generally by line <b>277</b> in <figref idrefs="DRAWINGS">FIG. 2</figref>. This may occur using, e.g., wand <b>155</b> (<figref idrefs="DRAWINGS">FIG. 1</figref>) to communicate by RF energy with implantable embodiments of MD <b>200</b>, such as generator <b>110</b> (<figref idrefs="DRAWINGS">FIGS. 1A-C</figref>). Alternatively, the wand may be omitted in some systems, e.g., systems in which monitoring unit <b>270</b> operates in the MICS bandwidths.

In one embodiment, the monitoring unit <b>270</b> may comprise a local database unit <b>255</b>. Optionally or alternatively, the monitoring unit <b>270</b> may also be coupled to a database unit <b>250</b>, which may be separate from monitoring unit <b>270</b> (e.g., a centralized database wirelessly linked to a handheld monitoring unit <b>270</b>). The database unit <b>250</b> and/or the local database unit <b>255</b> are capable of storing various patient data. This data may comprise patient heart beat and/or heart parameter data acquired from a patient's body, as well as therapy parameter data. The database unit <b>250</b> and/or the local database unit <b>255</b> may comprise data for a plurality of patients, and may be organized and stored in a variety of manners, such as in date format, severity of disease format, etc. The database unit <b>250</b> and/or the local database unit <b>255</b> may be relational databases in one embodiment. A physician may perform various patient management functions (e.g., programming parameters for a primary/chronic therapy and/or secondary/acute therapy modes) using the monitoring unit <b>270</b>, which may include obtaining and/or analyzing data from the MD <b>200</b> and/or data from the database unit <b>250</b> and/or the local database unit <b>255</b>. The database unit <b>250</b> and/or the local database unit <b>255</b> may store various patient data.

One or more of the blocks illustrated in the block diagram of the MD <b>200</b> in <figref idrefs="DRAWINGS">FIG. 2</figref> may comprise hardware units, software units, firmware units, or any combination thereof. Additionally, one or more blocks illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref> may be combined with other blocks, which may represent circuit hardware units, software algorithms, etc. Additionally, any number of the circuitry or software units associated with the various blocks illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref> may be combined into a programmable device, such as a field programmable gate array, an ASIC device, etc.

The implantable medical device system of one embodiment of the present invention provides for software module(s) that are capable of acquiring, storing, and processing various forms of data, such as patient data/parameters (e.g., physiological data, side-effects data, such as heart rate, breathing rate, brain-activity parameters, disease progression or regression data, quality of life data, etc.) and therapy parameter data. Therapy parameters may include, but are not limited to, electrical signal parameters that define the therapeutic electrical signals delivered by the IMD, medication parameters and/or any other therapeutic treatment parameter. In an alternative embodiment, the term “therapy parameters” may refer to electrical signal parameters defining the therapeutic electrical signals delivered by the IMD. Therapy parameters for a therapeutic electrical signal may also include, but are not limited to, a current amplitude, a pulse width, a frequency, an on-time, an off-time, etc.

In one embodiment, the present invention may include coupling of at least one electrode to each of two or more cranial nerves. (In this context, two or more cranial nerves mean two or more nerves having different names or numerical designations, and do not refer to the left and right versions of a particular nerve). In one embodiment, at least one electrode may be coupled to either or both vagus nerves or a branch of either or both vagus nerves. The term “operatively” coupled may include directly or indirectly coupling. Each of the nerves in this embodiment or others involving two or more cranial nerves may be stimulated according to particular activation modalities that may be independent between the two nerves.

In one embodiment, the MD <b>200</b> may also be capable of detecting a manual input from the patient. The manual input may include a magnetic signal input, a tap input, a wireless data input to the MD <b>200</b>, etc. In one embodiment, based upon the manual input, alterations in the treatment regimen may be provided for treating a chronic condition.

Turning now to <figref idrefs="DRAWINGS">FIG. 4A</figref>, a more detailed stylized depiction of the heart beat sensor interface <b>265</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>, in accordance with one illustrative embodiment of the present invention is depicted. In one embodiment, the heart beat sensor interface <b>265</b> comprises a heart beat signal receiver <b>410</b>, an analog-to-digital converter (A/D Converter) <b>420</b>, and a heart beat data correlation unit <b>425</b>. The heart beat signal receiver <b>410</b> is capable of receiving the signals from the sensor(s) <b>212</b>. The signal that is received by the heart beat signal receiver <b>410</b> is then processed and filtered.

The heart beat signal receiver <b>410</b> may comprise a receiver circuit <b>412</b>, amplifier(s) <b>414</b>, and filter(s) <b>416</b>. The receiver circuit <b>412</b> is capable of receiving input signal(s) from the sensor(s) <b>212</b>. The receiver circuit <b>412</b> may be a multi-channel receiver that is capable of receiving a plurality of input signals from a plurality of channels. The receiver circuit <b>412</b> may comprise circuitry for receiving input signal, such as interface circuitry. The receiver circuit <b>412</b> may provide the input signal to the amplifier(s) <b>414</b>. The amplifiers <b>414</b> are capable of buffering and amplifying the input signals received from the sensor(s) <b>212</b>. The heart beat signal may be attenuated and may be characterized by significantly low amplitude responses and signal noise. The amplifier(s) <b>414</b> are capable of buffering (amplification by unity) and amplifying the signals for further processing. In one embodiment, the amplifier <b>414</b> may comprise op amp circuit(s), digital amplifier(s), buffer amplifiers, and/or the like.

The heart beat signal receiver <b>410</b> may also comprise one or more filters <b>416</b>. The filters <b>416</b> may comprise analog filter(s), digital filter(s), filters implemented by digital signal processing (DSP) means or methods, etc. The amplified and buffered heart beat signal may be filtered to remove various noise signals residing on the heart beat signal. The filter <b>416</b>, for example, is capable of filtering out various noise signals caused by external magnetic fields, electrical fields, noise resulting from physiological activity, etc. For example, signal noise due to breathing, or other signals produced by the patient's body may be filtered.

The heart beat signal receiver <b>410</b> is also capable of filtering various other signal components, such as performing an R-R interval rejection, wherein R-R beat components introduced to the heart beat signal may be corrected using this filter. Further, sampling-rate induced errors in the heart beat signal may be filtered using the filter <b>416</b>. For example, a limited sampling rate may introduce an error into the heart rate variability (HRV) spectrum that may increase the frequency, particularly affecting higher frequency components. The filter <b>416</b> is capable of reducing or eliminating this effect.

The filter <b>416</b> may also be capable of performing an interpolation of the under-sampled ECG signal to decrease this error. Editing of R-R interval data may be performed on the heart beat signal. Certain R-R intervals may be eliminated to avoid unnecessary calculations. For example, R-R intervals that differ by more than a predetermined percentage, such as 20%, from the previous interval may be eliminated to maintain more accurate R-R interval signals, thereby performing the R-R data editing. The filter <b>416</b> may also perform artifact elimination due to various other components, such as breathing or other noise sources.

The heart beat signal receiver <b>410</b> provides amplified, filtered signals to the A/D converter <b>420</b>. The A/D converter <b>420</b> performs an analog-to-digital conversion for further processing of the heart beat signal. The A/D converter <b>420</b> may be one type of a plurality of converter types with various accuracies, such as an 8-bit converter, a 12-bit converter, a 24-bit converter, a 32-bit converter, a 64-bit converter, a 128-bit converter, a 256-bit converter, etc. The converted digital signal is then provided to a heart beat data correlation unit <b>425</b>. In an alternative embodiment, the A/D conversion may be performed prior to filtering or signal processing of the heart beat signal. The converted digital signal is then provided to a heart beat data correlation unit <b>425</b>.

The heart beat data correlation unit <b>425</b> is capable of organizing, correlating, stacking, and otherwise processing the digitized, buffered, and filtered heart beat data. The heart beat correlation unit <b>425</b> is capable of correlating and organizing the digitized heart beat signal. The correlation unit <b>425</b> may correlate various time stamps with the heart beat signal to provide a time of beat sequence of the patient's heart. Further, the heart beat data correlation unit <b>425</b> is capable of correlating various physiological events to the heart beat data. The digital signals issuing from the heart beat data correlation unit <b>425</b> may then be forwarded to the heart beat data processing module <b>275</b>.

Turning now to <figref idrefs="DRAWINGS">FIG. 4B</figref>, a more detailed stylized depiction of the heart beat data processing module <b>275</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>, in accordance with one illustrative embodiment of the present invention is depicted. The heart beat data processing module <b>275</b> may comprise a heart beat determination module <b>430</b>, a beat interval determination module <b>440</b>, a beat interval time series storage unit <b>450</b>, and a beat interval statistics module <b>460</b>. The heart beat data processing module <b>275</b> may determine heart beats as they appear in the time series of signals via the heart beat determination module <b>430</b>. For example, heart beat determination module <b>430</b> may characterize certain data points in the time series of signals as corresponding to the start, the peak, or the end of an R-wave of a patient's cardiac cycle.

Once heart beats are determined from the time series of signals, the beat interval determination module <b>440</b> may determine the interval between consecutive beats (“beat interval”) and forward this information to beat interval time series storage <b>450</b>. From the determined beat interval and/or the time series thereof, the beat interval statistics module <b>460</b> can determine various statistical values of the beat interval time series, e.g., mean, median, or standard deviation, among others, for various timescales (e.g., 5 minutes, 1 hour, 24 hours). The beat interval time series, the statistical values thereof, or both may be used for further processing.

Turning now to <figref idrefs="DRAWINGS">FIG. 5A</figref>, a more detailed stylized depiction of the HRV module <b>285</b> within heart parameter module <b>284</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>, in accordance with one illustrative embodiment of the present invention is depicted. The HRV module <b>285</b> may receive various data from the heart beat data processing module <b>275</b>. Based upon data from the heart beat data processing module <b>275</b>, the HRV module <b>285</b> is capable of determining at least one HRV parameter that is associated with a parasympathetic function of the patient, and performing further calculations in light of the HRV parameter, which may lead it to provide information to the controller <b>210</b>. In one embodiment, the HRV module <b>285</b> is capable of determining that a treatment should be provided to treat a chronic or non-episodic disorder. This indication may be stored internally and/or externally, e.g., in the memory <b>217</b> (<figref idrefs="DRAWINGS">FIG. 2</figref>). This indication may also be transmitted to a separate device, e.g., to the monitoring unit <b>270</b> and/or stored, e.g., into the local database unit <b>255</b> and/or the database unit <b>250</b> (<figref idrefs="DRAWINGS">FIG. 2</figref>). Heart module <b>284</b> and/or HRV module <b>285</b> may initiate other responsive actions such as providing an audible, visible, or tactile alert to the patient or a caregiver; logging a timestamp of the determination of a need for treating the chronic medical condition; initiation of a severity determination routine based upon data from the heart beat data processing module <b>275</b> and/or the HRV module <b>285</b> itself, communicating with one or more of database unit <b>250</b> or remote device <b>292</b>, or notifying emergency services via email, autophone communications, wireless communications, etc. It may be appreciated that, based upon the output of the heart parameter module <b>284</b>, responsive action(s) may be performed by either the MD <b>200</b>, monitoring unit <b>270</b>, or other devices such as remote device <b>292</b>.

The beat interval time series, its statistical values, or both are analyzed by an HRV parameter calculation module <b>510</b>, which determines the value of the at least one HRV parameter of interest. The HRV parameter calculation module <b>510</b> may store results in HRV parameter time series storage <b>520</b>, which may be a portion of the memory <b>217</b> or a separate memory unit. The HRV parameter calculation module <b>510</b> may also access information from HRV parameter time series storage <b>520</b> to calculate one or more HRV parameters. The HRV parameter time series storage <b>520</b> may communicate HRV parameter time series information to a monitoring unit <b>270</b> via communications unit <b>260</b>.

After calculating the at least one HRV parameter of interest (module <b>510</b>), the calculated value may be compared to a threshold value by an HRV parameter/threshold comparison module <b>530</b>. The threshold value used by the module <b>530</b> may be stored in HRV parameter threshold storage <b>540</b> after being placed there by a physician via communications unit <b>260</b>. The threshold value used by the module <b>530</b> may be a portion of the memory <b>217</b> or a separate memory unit. In one embodiment, the HRV parameter threshold storage module <b>540</b> may calculate a threshold value to provide an adaptive threshold rather than a fixed threshold. For example, thresholds may be calculated from a baseline HRV value for a particular patient that is determined from data stored in HRV parameter time series storage <b>520</b>, or other algorithms for determining a threshold may be implemented. In another embodiment, an HRV parameter threshold may be modified based upon circadian rhythms of the patient.

Turning now to <figref idrefs="DRAWINGS">FIG. 5B</figref>, a more detailed stylized depiction of the regulatory state module <b>287</b> of <figref idrefs="DRAWINGS">FIG. 2</figref>, in accordance with one illustrative embodiment of the present invention is depicted. The regulatory state module <b>287</b> may receive various data from the heart beat data processing module <b>275</b>. Based upon data from the heart beat data processing module <b>275</b>, the regulatory state module <b>287</b> is capable of determining at least one regulatory state parameter, such as those described above (e.g., IARS), and performing further calculations in light of the regulatory state parameter, which may lead it to provide information to the controller <b>210</b>. In one embodiment, the regulatory state module <b>287</b> is capable of determining that a treatment should be provided to treat a chronic or non-episodic disorder. This indication may be stored internally and/or externally, e.g., in the memory <b>217</b> (<figref idrefs="DRAWINGS">FIG. 2</figref>). This indication may also be transmitted to an external entity, e.g., to the monitoring unit <b>270</b> and/or stored, e.g., into the local database unit <b>255</b> and/or the database unit <b>250</b> (<figref idrefs="DRAWINGS">FIG. 2</figref>). Module <b>287</b> may also initiate other responsive actions such as those described for HRV module <b>285</b> in <figref idrefs="DRAWINGS">FIG. 5A</figref>, supra.

The beat interval time series, its statistical values, or both are analyzed by a regulatory state parameter calculation module <b>550</b>, which determines the value of the at least one regulatory state parameter of interest. The regulatory state calculation module <b>550</b> may store results in regulatory state parameter time series storage <b>560</b>, which may be a portion of the memory <b>217</b> or a separate memory unit. The regulatory state parameter calculation module <b>550</b> may also access information from regulatory state parameter time series storage <b>560</b> to calculate one or more regulatory state parameters. The regulatory state parameter time series storage <b>560</b> may communicate regulatory state parameter time series information to a monitoring unit <b>270</b> via communications unit <b>260</b>.

After module <b>550</b> calculates the at least one regulatory state parameter of interest, the value may be compared to a threshold value by a regulatory state parameter/threshold comparison module <b>570</b>. The threshold value used by the module <b>570</b> may be stored in a regulatory state threshold storage <b>580</b> after being placed there by a physician via communications unit <b>260</b>. The threshold value used by the module <b>570</b> may be a portion of the memory <b>217</b> or a separate memory unit. As with the HRV threshold value, module <b>580</b> may also calculate an adaptive threshold rather than a fixed threshold, based upon a baseline regulatory state parameter, environmental or patient-specific parameters, or combinations of the foregoing.

Turning now to <figref idrefs="DRAWINGS">FIG. 6</figref>, a flowchart depiction of steps involving processing heart rate variability (HRV) data, in accordance with one illustrative embodiment of the present invention, is provided. Heart beat data is received by the MD <b>200</b> (block <b>610</b>). In one embodiment, HRV data is used to determine an NN data sequence. In one embodiment, the variations in heart beat data may be evaluated based upon time domain analysis (block <b>630</b>). In another embodiment, the variations in heart beat data may be evaluated based upon a frequency domain analysis (block <b>650</b>). In still other embodiments, the variations in heart beat data may be subjected to rhythm pattern analysis (block <b>660</b>) and/or nonlinear HRV analysis (block <b>670</b>).

Based upon data from an ECG, the QRS complex may be evaluated. Based upon the QRS complex, normal-to-normal (NN) intervals may be determined (block <b>620</b>). As noted above, NN intervals may refer to intervals between adjacent corresponding QRS complexes. Time domain variables may then be calculated based upon the NN intervals (block <b>630</b>). Examples of the time domain variables may include mean NN intervals, instantaneous heart rate, the difference between the longest and shortest NN intervals, the difference between heart rate for different time periods, etc. Direct measurements of the NN intervals, instantaneous heart rate, or NN determination based upon differences between the intervals may be analyzed. A standard deviation of the NN intervals may be made to provide a standard deviation NN parameter (SDNN), which may reflect cyclic components relating to the variability in the heart rate signals for a given time period. The SDNN is an example of a statistical parameter.

The NN data sequence is used to perform the time domain analysis of the HRV to provide various parameters that may be indicative of a patient's regulatory system. The NN data sequence may also be used to perform interpolation and data sampling to perform a frequency domain analysis of the HRV (block <b>640</b>). A spectral method, such as a power spectral density (TSD) analysis may provide information relating to the power (variance) distribution as a function of frequency. Algorithms, such as fast Fourier transform (FFT) may be performed for such analysis. Parameters such as total power for a particular time period may be calculated to provide an indication of the variance of the NN intervals over a time period. The HRV signals may be analyzed to provide a calculation of low frequency (LF) and high frequency (HF) power components with emphasis upon a central frequency of each component. Based on the time domain HRV analysis and/or the frequency domain HRV analysis, various indications of heart rate variability parameters for detecting regulatory system operation of the patient is possible. The HRV analysis of the patient's regulatory system includes one or more of a statistical parameter, a spectral parameter, a fractal parameter, and/or a non-linear parameter. Examples of statistical parameter may include the SDNN, the SDANN, the RMSSD, and PNN50, among others. Examples of the spectral parameter may be an LF power, an HF power, and/or a ratio thereof, among others. Examples of a non-linear parameter may be a Lyapunov exponent or a Kolmogorov exponent, as described above, among others.

Turning now to <figref idrefs="DRAWINGS">FIG. 7</figref>, a circuit type model of the parasympathetic interaction with the heart rhythm is illustrated. As noted above, HRV data may be viewed as relating to one or more regulatory mechanisms influencing the circulatory system of a patient. As illustrated in <figref idrefs="DRAWINGS">FIG. 7</figref>, the cerebral cortex (block <b>710</b>) interfaces with the autonomic centers and the subcortical nerve centers (block <b>720</b>) and with the cardiovascular control center (block <b>730</b>) in the medulla oblongata. Blocks <b>720</b>-<b>730</b> make up the central circuit of the regulation of the heart beat, which is controlled by the neurohumoral regulation of the physiological functions.

<figref idrefs="DRAWINGS">FIG. 7</figref> also illustrates an autonomic circuit, which may include the sinus node <b>740</b> and the vagus system (block <b>750</b>), which includes the vagus nuclei in the medulla oblongata. The autonomic circuit also includes a feedback component, i.e., the respiratory system (block <b>760</b>) which includes the lungs and the respiratory control center. A connection between the central circuit and the autonomic circuit may be provided by the nervous regulation channel.

Stress upon the system of <figref idrefs="DRAWINGS">FIG. 7</figref> may be exacerbated by a chronic medical condition, and may be characterized using the disclosures of the present application. This characterization may be used as a triggering mechanism to provide improved treatment methods for chronic conditions. Certain chronic disorders may affect, and be affected by, the parasympathetic activity of the body. The HRV parameters exemplified herein may be used as a triggering mechanism for aiding a determination whether to treat the chronic conditions that cause adverse effects upon the parasympathetic system of the patient's body. For example, a therapeutic stimulation signal may be applied to a portion of the vagus system (e.g., a portion of the vagus nerve) to affect a patient's parasympathetic system, which may thereby affect the interaction between the autonomic circuit and the central circuit, and their respective functions. This modulation of the parasympathetic system may result in treatment of a chronic or non-episodic condition or disorder. By providing treatment in response to detection of HRV or regulatory system parameters exceeding acceptable thresholds, it is believed that improved treatments for the chronic medical condition may be provided.

Turning now to <figref idrefs="DRAWINGS">FIG. 8</figref>, a stylized flowchart depiction of providing treatment of a chronic medical condition by the MD <b>200</b>, in accordance with one illustrative embodiment of the present invention, is provided. The MD <b>200</b> receives a heart beat signal (block <b>810</b>). The heart beat sensor interface <b>265</b> in the MD <b>200</b> receives the heart beat signal. After performing buffering, amplification, filtering, and A/D conversion of the heart beat signal, the heart beat data processing module <b>275</b> processes the heart beat data for performing analysis of heart beat data (block <b>820</b>). The processing of the heart beat data may be performed by the heart beat data processing module <b>275</b> (<figref idrefs="DRAWINGS">FIG. 2</figref>).

Based upon the processed HRV data, a step for determining an IARS parameter (block <b>830</b>) and/or at least one HRV parameter (block <b>840</b>) are determined. As noted above, the determination of the regulatory parameter or the HRV parameter(s) of the patient may be analyzed. In particular, an IARS (regulatory) parameter or any of numerous HRV parameters (e.g., a statistical parameter, a spectral parameter, and/or a non-linear parameter) may be determined. Based upon the parameters that relate to the regulatory system of the patient, a determination may be made as to whether the regulatory and/or HRV parameter(s) indicate a need for treatment of a chronic medical condition of the patient. (block <b>850</b>). Some embodiments of the present invention are directed to treating chronic conditions that adversely affect the patient's parasympathetic system. Accordingly, the relationship between the chronic medical condition and the parasympathetic system of the patient may be determined. A more detailed description of the steps for determining whether the regulatory and/or HRV parameters indicate a need for treatment (block <b>850</b>) is further described in <figref idrefs="DRAWINGS">FIGS. 8 and 9</figref> and accompanying description below.

Upon determining that the regulatory and/or HRV parameter(s) indicates a need for treatment of the chronic condition, the MD <b>200</b> may then treat the chronic condition (block <b>860</b>). This treatment may include providing an electrical signal therapy to a portion of the vagus nerve to affect the parasympathetic system of the patient's body.

Referring now to <figref idrefs="DRAWINGS">FIG. 9</figref>, an algorithm is provided to illustrate how a determined IARS parameter (i.e., a regulatory parameter) may be used to determine whether a chronic medical condition of a patient should be treated. A comparison of the determined IARS parameter to a first threshold IARS value may be performed (block <b>910</b>). A determination is made whether the determined IARS value is greater than a first IARS threshold value (block <b>920</b>). In the event that a determination is made that the determined IARS value is not greater than the first IARS threshold value (block <b>922</b>), no application of therapy is performed at that time (blocks <b>930</b>).

In a similar manner, a determination may be made whether the IARS is greater than a second IARS threshold (block <b>940</b>). If the IARS parameter is not greater than the second threshold (block <b>942</b>), then a first therapy is applied (e.g., an electrical signal therapy applied to a vagus nerve) to the patient (block <b>950</b>). In this manner, if the IARS is greater than the first IARS threshold, but not greater than the second IARS threshold, a first therapy signal is applied. This process may be repeated numerous times wherein a comparison is made whether the IARS is greater than an n^thIARS threshold (block <b>960</b>). Upon a determination that the IARS parameter is not greater than the n^ththreshold, an (n−1)^ththerapy signal (e.g., an electrical signal applied to a vagus nerve) is applied to the patient (block <b>970</b>). However, if the IARS parameter is greater than or equal to the n^ththreshold, then the n^ththerapy is applied (block <b>980</b>). In this manner, depending on the value of the IARS parameter, one of a number of electrical signal therapies may be applied. Therefore, depending on the stress, tension, instability, or complete exhaustion of the regulatory system of the patient, particular therapies or interventions may be applied to treat the patient's chronic medical condition.

Referring now to <figref idrefs="DRAWINGS">FIG. 10</figref>, an algorithm is provided to illustrate how a determined HRV parameter may be used to determine whether a chronic medical condition of a patient should be treated. A comparison of a determined HRV parameter to a first predetermined threshold HRV parameter value may also be performed (block <b>1010</b>). A determination is made as to whether a determined HRV parameter is less than a first threshold HRV parameter value (block <b>1020</b>). In the event that a determination is made that the determined HRV parameter value is not greater than the first threshold HRV parameter value (block <b>1022</b>), no application of therapy is performed at that time (blocks <b>1030</b>).

Similarly, a determination may be made as to whether the determined HRV parameter value is less than a second HRV threshold value (block <b>1040</b>). If the HRV parameter is not less than the second HRV parameter threshold (block <b>1042</b>), then a first therapy is applied to the patient (block <b>1050</b>). In this manner, if the HRV parameter is less than the first HRV threshold, but not less than the second HRV threshold, a first therapy signal is applied. This process may be repeated numerous times wherein a comparison is made whether the HRV is less than an n^thHRV threshold (block <b>1060</b>). Upon a determination that the HRV parameter is not less than the n^ththreshold, an (n−1)^ththerapy signal is applied to a portion of a cranial nerve (e.g., vagus nerve) of the patient (block <b>1070</b>). However, if the HRV parameter is less than or equal to the n^ththreshold, then the n^ththerapy is applied (block <b>1080</b>). In this manner, depending on the value of the HRV parameter, one of a number of electrical signal therapies may be applied. Therefore, depending on the stress, tension, or instability of the regulatory system of the patient, particular therapy signals may be applied to treat a chronic medical condition. Utilizing the techniques described herein, various types of chronic conditions or non-episodic conditions may be treated.

All of the methods and apparatuses disclosed and claimed herein may be made and executed without undue experimentation in light of the present disclosure. While the methods and apparatus of this invention have been described in terms of particular embodiments, it will be apparent to those skilled in the art that variations may be applied to the methods and apparatus and in the steps, or in the sequence of steps, of the method described herein without departing from the concept, spirit, and scope of the invention, as defined by the appended claims. It should be especially apparent that the principles of the invention may be applied to selected cranial nerves other than, or in addition to, the vagus nerve to achieve particular results in treating patients having epilepsy, depression, or other medical conditions.

The particular embodiments disclosed above are illustrative only as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown other than as described in the claims below. It is, therefore, evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.